1. Technical Field
This invention relates to digital televison (DTV) and, more particularly, to the provision of an ultra wideband overlay signal in the DTV transmission for mitigation of multipath.
2. Background Art
High definition television (HDTV) is an emerging technology that is capable of providing service either in an analog or digital format. In the United States, research on HDTV has focused on digital, rather than analog, technology. While digital HDTV is not currently available, Japanese companies have developed an HDTV system based on analog technology (known as Hivision) that has been in use since 1991. Because of the potential advantages of digital HDTV and many technical problems shared by both types of systems, research in digital HDTV has also been active in Japan. See, for example, David K. Kahaner in xe2x80x9cHDTV Research in Japanxe2x80x9d, IEEE Micro, October 1993, pp. 49-53.
One of the most important prevalent problems in digital television (DTV) is the problem of multipath. In fact, it is useful to think of the DTV channel as multipath limited and not power limited. Multipath may arise from fixed structures acting as reflectors in the transmission channel such as building walls. Moving objects, such as airplanes, may also cause a multipath condition. Even microreflections in cabling can cause multipath. See, for example, P. T. Marhiopoulos and M. Sablatash, xe2x80x9cDesign of a Ghost Canceling Reference Signal for Television Systems in North Americaxe2x80x9d, Proceedings of Canadian Conference on Electrical and Computer Engineering, Vancouver, BC, Canada, Sep. 14-17, 1993, pp. 660-663.
The effect of multipath is to create xe2x80x9cghostsxe2x80x9d in the displayed TV image. The statistics of multipath ghosts have been studied and compiled by, among others, the BTA (Japan""s Broadcasting Technology Association). A BTA survey reported that 92% of ghosts are within a xe2x88x924 to 26 microsecond range, and when extended to xe2x88x924 to 37 microseconds, almost all ghost occurrences are covered.
An adaptive equalizer has been proposed to xe2x80x9cundoxe2x80x9d the effects of the multipath. In its crudest form, an adaptive equalizer functions as a signal processor for estimating the parameters of a hypothetical filter that best describes the channel. The signal processor adjusts the taps of the adaptive equalization filter to approximate an inverse of the hypothetical filter, thus inverting or undoing the effects of the multipath.
The BTA, and other concerns, designed a xe2x80x9cghost canceling reference (CGR)xe2x80x9d transmitted signal to mitigate these multipath induced effects. The BTA GCR was found to be less than satisfactory in some instances. While homes with outdoor antennas displayed non-varying (stationary) ghosting conditions which could be largely corrected, those homes with indoor antennas experienced changing (dynamic) ghosts. These dynamic ghosting conditions were more prevalent where people were moving about the room or other moving objects were in the signal path. The BTA ghost canceller generally could not adequately compensate for these conditions. In fact, false ghosts were actually added to an already ghosted picture, leading to reduced picture quality.
Thus, multipath behavior of the DTV channel is important for two different regimes, the outdoor antenna propagation channel and the indoor antenna propagation channel. The former is well-studied and understood. The latter question still presents a problem. The chief difference is the presence of significant reflectors near the indoor receiving antenna, which implies that there will be multipath whose delay occasions it to fall within a symbol period. In order to resolve multipath differences of such limited extent, special techniques must be employed or the channel diagnostic signal must have a very wide effective bandwidth. According to S. Salous in xe2x80x9cIndoor and Outdoor UHF Measurements with a 90 MHz Bandwidthxe2x80x9d, IEEE Colluquium on Propagation Characteristics and Related System Techniques for Beyond Line-of-Sight Radio, 1997, pp. 8/1-8/6, the extent of multipath delays in outdoor environments can be a few tens of microseconds, whereas in indoor environments, it is on the order of a few hundred nanoseconds. While multipath components can be adequately resolved with a 10 to 40 MHz bandwidth for outdoor environments, the resolution of multipath for indoor environments requires a 90 to 100 MHz bandwidth.
Additional xe2x80x9cchannel informationxe2x80x9d is provided to a consumer-grade digital television receiver within the context of the adopted ATSC (Advanced Television Standards Committee) standard for digital broadcast. By xe2x80x9cchannel informationxe2x80x9d is meant a transfer function effectively describing the relation between the transmitted signal and the received signal. This transformation may be a series of time-delayed and amplitude-attenuated replicas of the same signal, called multipath. However, the transformation may be much more complicated, involving diffraction, refraction, and polarization effects. Using digital signal processing, knowledge of the channel information at the receiver can be exploited to improve the digital television receiver design. Exploitation of the channel information in the receiver design can make the critical difference between being able to receive or not receive a particular digital TV channel in a home. From a broadcaster""s perspective, exploitation of the channel information means increased coverage area, hence an increased number of viewers and ultimately increased revenue.
The DTV transmission is thus provided with an ultra wideband overlay signal which is used by suitably equipped DTV receivers to dynamically estimate the channel and allow expeditious and effective mitigation of changing multipath conditions. A DTV receiver not equipped to process the overlay transmission will not be significantly affected by the overlay signal; that is, the invention is backwardly compatible.
The ultra wideband overlay signal is typically on the order of 90 to 100 MHz in bandwidth to enable characterization of the indoor multipath. This ultra wideband overlay signal differs from the wideband overlay signal described in copending patent application Ser. No. 09/201,376 in that it overlaps many other transmission frequencies. This ultra wideband overlay signal is designed in cooperation with other such wideband overlay signals so as to exhibit low cross-correlation with the wideband overlay signal of the invention described in application Ser. No. 09/201,376, which has relatively little power outside of the channel carrying the associated DTV signal. Because of its rate and sequence length, implementation of the ultra wideband overlay signal is built out of a sequence that can be sequentially synchronized, such as Titsworth""s component codes or xe2x80x9cJPL codesxe2x80x9d.